We now have GTK_CSS_AFFECTS_CONTENT for properties that have an effect
on content rendering.

Using GTK_CSS_AFFECTS_ICON is wrong for icon-transform and icon-filter
as they don't change the icon, just how the icon is rendered, so we use
GTK_CSS_AFFECTS_CONTENT for those.

We also introduce GTK_CSS_AFFECTS_POSTEFFECT for opacity and filter -
properties that affect the whole drawing of the widget by applying an
effect after everything is said and done.